Entropy-Driven Depolymerization of Poly(dimethylsiloxane)

Abstract

Poly(dimethylsiloxanes) (PDMSs) are widely used because of their unique properties but require a highly polluting and energy-intensive synthesis starting from silica. Chemical recycling offers an opportunity to regenerate new materials with comparable properties while avoiding the inefficiencies of the de novo synthesis. Herein, we report computational and experimental results of depolymerizing PDMS with diols. Computationally, depolymerization with methanol is always endergonic, while depolymerization with 2-methyl-2,4-pentanediol (hexylene glycol, HG) is exergonic over a wide temperature range. Acid-catalyzed exchange reactions between SiMe2(OMe)2 and hexylene glycol show silicon’s thermodynamic preference for the diol due to entropically favorable chelation. An optimized procedure for depolymerizing PDMS with hexylene glycol to a cyclic monomer, M2HG, was developed and applied to a variety of commercial PDMS sources. Cyclic siloxanes are byproducts early in the reaction, requiring a two-step process to obtain highly pure M2HG. The procedure is selective for PDMS even in complex reaction mixtures and gives good yields (38–78%) regardless of the starting material used. Polymerizing M2HG back to PDMS proved challenging but feasible. Ring-opening polymerization of M2HG was catalyzed by trifluoromethanesulfonic acid or p-toluenesulfonic acid to yield low-molecular-weight polysilicon acetals (Mn up to 1500) or high-molecular-weight PDMS (Mn ∼ 39,900), respectively. Terpolymerizations of M2HG with vinyl ethers and aldehydes also yielded low-molecular-weight material (Mn up to 5000). PDMS was successfully reconstituted from M2HG under cationic emulsion polymerization conditions, affording high molecular weights up to Mn = 49,000.